1. Field of the Invention
This invention relates to a molded surface fastener in which a multiplicity of hooks are molded on a substrate sheet by extrusion or injection molding using thermoplastic synthetic resin, and more particularly to a molded hook structure which has both adequate softness and strength of monofilament and is very durable.
2. Description of the Related Art
Surface fasteners of the type in which hooks are formed by weaving monofilaments in a woven cloth so as to form loop piles of monofilaments and then cutting the loop piles are well known in the art. This type surface fastener has softness of a woven cloth and softness of monofilament and is characterized in that the hooked surface fastener comes into engagement with and are peeled off loops of a companion surface fastener with a very smooth touch. Since the monofilaments constituting the hooks are treated by drawings, the surface fastener is excellent in pulling strength and bending strength even in a small cross-sectional area. Further, since the surface fastener can have a very high density of hooks depending on the woven structure, it is possible to secure a high engaging rate and an adequate degree of durability. However, with the woven type surface fastener, since consumption of material and a number of processing steps are large, it is difficult to reduce the cost of production.
For an improvement, a molded type surface fastener was developed in which a substrate sheet and hooks are formed integrally and simultaneously by extrusion or injection molding. A typical example of molding technology for this type surface fastener is disclosed in, for example, U.S. Pat. No. 3,312,583 and WO 87/06522. As a rotary drum in which a number of molding disks each having on an outer peripheral edge of each of opposite surfaces a number of hook-forming cavities and a number of spacer disks each having flat surfaces are alternately superimposed one over another is rotated, molten synthetic resin material is forced against its peripheral surface to fill the cavities and then the hooks formed in the cavities are removed off the drum along with the substrate sheet. The spacer disks are disposed between the molding disks because the cavities of the whole shape of the hooks cannot be made in one mold due to the shape of the hooks.
However, in the molded type surface fastener, partly since a delicate shape cannot be obtained as compared to the woven type surface fastener due to technical difficulty in molding process, and partly since the formed hooks are poor in orientation of molecules, only a very low degree of strength can be achieved with the same size of the above-mentioned monofilament hooks. Therefore none of the conventional molded type surface fasteners are satisfactory for practical use. Further, according to the conventional hook structure, the individual stem is simple in cross-sectional shape and would hence tend to fall flat from its base. As a result, the individual stems would not restore their original posture after repeated use, thus lowering the rate of engagement with loops of a companion surface fastener. Therefore, in order to secure desired strength, it is absolutely necessary to increase the size of the individual hooks, which makes the hooks rigid and the number of hooks per unit area (density of hooks) reduced to lower the rate of engagement with the companion loops.
As a solution, a new hook structure which enables a smooth touch, with the stem hardly falling flat, during the engaging and peeling operation likewise the woven type surface fastener and which increases the rate of engagement to secure adequate strength and durability is disclosed in, for example, U.S. Pat. No. 5,131,119. In the molded type surface fastener disclosed in this U.S. Patent, each hook has a hook-shape engaging portion extending forwardly from the distal end of a stem, which has a rear surface rising obliquely in a smooth curve from a substrate sheet and a front surface rising upwardly from the substrate sheet, and a reinforcing rib projecting from a side surface of the stem, the cross-sectional area of the hook increasing gradually from a tip of the hook-shape engaging portion toward the base of the stem. The reinforcing rib serves to prevent the stem from falling laterally and also to minimize the stem and the hook-shape engaging portion while maintaining a required degree of engaging strength. The height of the reinforcing rib is substantially equal to a half of the height of the tip of the hook-shape engaging portion.
U.S. Pat. No. 5,339,499 also discloses a hook structure in which a reinforcing rib having the same thickness as that of a hook and extending upwardly beyond the height of the tip of a hook-like engaging portion is situated on one side surface of the hook.
However, according to U.S. Pat. No. 5,131,119, because of the above-mentioned shape of the reinforcing rib, both a hook-side surface and a rear surface of the hook-shape engaging portion above the apex of the reinforcing rib are subject to large local force due to being stretched and depressed. Therefore, if the force is exerted to the hook-shape engaging portion repeatedly, a portion around the apex of the reinforcing rib becomes weak and the hook tends to get cut at that portion.
The shape of the reinforcing rib disclosed in U.S. Pat. No. 5,339,499 is identical with the shape of the stem of the hook as seen in side view. The whole shape of the hook corresponds to the shape in which one of laterally divided halves of a single hook-shape engaging portion is removed. Specifically, in the conventional molded hook structure devoid of a reinforcing rib, the hook except the stem and a base of the hook-shape engaging portion is laterally divided into halves, and one half is removed to reduce the thickness of the hook-shape engaging portion. Thus the hook-shape engaging portion is reduced to half in thickness and is hence deformable while the stem has the same degree of rigidness as conventional.
Application of this kind of molded surface fasteners is on the increase for use in paper diapers and underwear for babies, and hooks having improved softness are cherished. There is a limit in selecting the material for such softness, and adequate softness should necessarily depend on a rational structure of the hook.
However, in the hook structure disclosed in U.S. Pat. No. 5,131,119, if the hook is too thin, the reinforcing rib has only a too small height so that the hook is too soft not only in the hook-shape engaging portion but also in the joint between the stem and the hook-shape engaging portion. The hook is accordingly tends to flex at the joint to lower the rate of engagement with the loops to the utmost.
Further, in the hook structure disclosed in U.S. Pat. No. 5,339,499, the reinforcing rib is identical in side shape with the hook throughout the stem and part of the hook-shape engaging portion, the almost whole of the hook has the same thickness as conventional. It is therefore impossible to avoid a rigid touch when the user's skin comes into direct or indirect contact with the hooks. Also, since the entire hook except the hook-shape engaging portion has a uniform thickness, in order to make the hook in whole flexible, it is inevitable to reduce the thickness of the stem, which is double the thickness of the hook-shape engaging portion, and the thickness of part of the hook-shape engaging portion as well as the thickness of the majority of the hook engaging portion. The resulting hook is very apt to fall flat sideways, lowing the rate of engagement with the companion loops to the utmost.